The invention relates to heated hair styling appliances. In particular, the invention pertains to hair styling appliances having heaters and control circuits therefor in which stable, safe and effective operating temperatures may be maintained for relatively long periods under variable heavy load conditions and which have relatively fast recovery times. The hair styling appliances to which the invention is applicable includes curling irons which are discussed in detail as well as hair straighteners, hair crimpers and hot combs.
Human hair has a number of properties which affect its susceptibility to taking a curl. These properties include hair thickness, texture and color. For example, thick dark hair is more difficult to curl than light thin or fine hair. So called ethnic hair, as it is generally known in the art, is particularly difficult to curl. It is generally thick and may already hold a natural curl which must first be relieved. In preparation for curling, the hair is wet and is treated with a styling aid to effect a desired precurling condition. Such pretreated hair may require the application of a relatively high heat for about 30 seconds in order to result in the desired curl. This represents a relatively high thermal load on the heater.
Although an effective curling temperature range is about 140.degree.-160.degree. C., known curling irons suitable for treating ethnic hair are generally over powered. These irons operate in a temperature range having a high temperature in excess of 300.degree. C. Such a high temperature is thought to be necessary in order to compensate for the high thermal load. However, even when operated at such a high temperature, only about three curls may be formed before the curling iron cools below the effective curling temperature, after which the iron must be allowed to recover. The recovery time may be as much as 10 minutes to reach the peak operating temperature whereupon curling may be resumed.
At the high temperature, extreme care must be exercised in order to prevent damaging the hair. When, as desired, lower temperatures are used, the irons have a shorter operating time and thus must be allowed to recover more frequently. Accordingly, it is not unusual for an operator to alternately employ more than one iron at a time.
Some curling irons are equipped with devices for limiting the maximum temperature in order to prevent damage of the hair or damage to the curling iron itself. Such devices suffer from inefficient operating cycles. Some known devices include a thermostat for controlling temperature. Thermostats are effective but are slow to react and cause irregular recovery or hysterises so that low temperature recovery is not effective. When the set point of the thermostat is reached, the device reacts with a change in voltage which is at the high point allowed. At this point, however, the temperature will overshoot. The opposite is also true that when the temperature falls, it drops below the desired point. Thus, because of its nature, when the thermostat reacts it allows a much higher or lower temperature level than desired.
Some systems use a positive temperature coefficient (PTC) device such as a thermistor having a resistance characteristic which changes with temperature to control high temperature cutoff. In a conventional PTC device, at some temperature known as the point of inflection, the resistance characteristic suddenly changes and becomes extremely steep. This effectively cuts input power into the heater. However, the response of the PTC may be greatly affected by the input power. As a result, conventional PTCs are difficult to control and are somewhat unstable because the resistance change is virtually unregulated. Such instability results in control problems and unsatisfactory results.
Overall, conventional circuits employing PTCs are often undesirably complex and expensive to manufacture. They require more components. Certain components are individually complex, e.g. IC chips. Some require high power and complex control circuits, e.g. SCRs. Accordingly, conventional circuits are more difficult to design and manufacture, and are not cost effective in many applications.
In addition to the hair styling applications herein described, for example, other applications in the consumer market which use heaters, such as, personal care items, food preparation appliances and tools may benefit from more cost effective, reliable, and safe heater controls with fast recovery times.